The present invention relates to photometers and more particularly to a photometer with an improved light source.
A photometer is a device used to measure the concentration of a component (chromophore) in a fluid (gas or liquid) by determining the absorbance of a specific wavelength of light by the chromophore. The concentration of the chromophore is determined from the measured absorbance through the Beer-Lambert relationship: A=ebc where A=absorbance, e=chromophore absorptivity, b=pathlength, and c=chromophore concentration.
One of the most common conventional photometers is a Sigrist photometer, which includes a light source, a filter wheel, a sample cell and a photoreceiver or detector. The light source generates a continuous beam of electromagnetic radiation (light) that follows an optical path that extends through lenses, passes through the filter wheel, travels through the sample cell and impinges on the detector. The filter wheel includes a reference filter and a measure filter. The reference filter is operable to isolate the light beam to a reference wavelength, while the measure filter is operable to isolate the light beam to a measure wavelength. The reference wavelength is one in which the chromophore has little or no absorption, while the measure wavelength is the one in which the chromophore absorbs energy. A chopper motor rotates the filter wheel to alternately pass the reference filter and the measure filter through the light beam, thereby converting the light beam into alternating measurement and reference beams.
From the filter wheel, the measurement and reference beams pass through a lens where they are collimated before passing through the sample cell. The beams exit the sample cell and are focused on the detector by another lens. The detector converts the beams into voltage signals proportional to the light intensity received by the detector. An electronics assembly calculates a ratio of the intensities of the measurement and reference beams, which eliminates errors resulting from luminosity fluctuations of the light source and any changes in the detector sensitivity. This transmittance ratio is then used by the electronics assembly to determine absorbance, which is the negative logarithm of transmittance. The absorbance, in turn, is used to calculate the concentration of the chromophore.
The light source for a photometer must generate a light beam that is sufficiently powerful for easy detection and measurement. In addition, its output power should be stable for reasonable periods. Light sources can be a continuum source, which emits radiation that changes in intensity only slowly as a function of wavelength, or a line source, which emits a limited number of spectral lines. Commonly used continuum light sources include tungsten/halogen lamps and deuterium lamps. A typical tungsten/halogen lamp provides a distribution of wavelengths from 320 to 2500 nm, while a typical deuterium lamp provides a distribution of wavelengths about 160 nm to about 350 to 400 nm. Both tungsten/halogen and deuterium lamps operate at high temperatures and consume a large amount of electric power. Moreover, the filter wheel and chopper motor introduce complexity into the operation of the photometer.
The present invention is directed to an improved photometer that addresses these issues.